1. Field of the Invention
The present invention relates to golf balls excellent in durability and feel at impact
2. Description of the Background Art
Conventionally, a golf ball having a thread-wound layer formed on a liquid center and a balata cover covering thereof has widely been used by advanced golfers and professional golfers as being excellent in shot feel and controllability. Since such a golf ball, however, has a structure that makes the manufacturing process complicated, and is poor in durability and resistance against abrasion, a variety of golf balls excellent in these characteristics as a whole have been developed.
In particular, an attempt to improve golf balls in durability is made by blending an increased amount of zinc acrylate, zinc methacrylate or any other similar co-crosslinker to provide an increased cross-linking density to absorb energy more effectively. On the other hand, feel at impact is improved by a proposed technique allowing a core to have a hardness profile flattened in the direction of the depth as seen from the surface. In this technique, it is hardened outward as seen from the center. The hardness, however, disadvantageously varies, and at impact a soft portion experiences stress-concentration, resulting in reduced durability.
Conventionally a variety of proposals have been made to generally improve various characteristics including durability and feel. Japanese Patent Laying-Open No. 59-91973, for example, proposes a golf ball having a solid core having a diameter of 36.0–38 mm, formed of rubber or resin containing at least 5 wt % of a carbon fiber, a silicon carbide fiber, or a boron fiber of 0.1 mm or more in length to improve the durability and feeling at the time of hitting.
Japanese Patent Laying-Open No. 62-64378 proposes a cover composition for a golf ball where a fine fiber having an amide group is blended in a cover material containing trans-polyisoprene as a main component to improve rebound property, spin property and cut resistance.
Japanese Patent Laying-Open No. 63-9461 proposes a cover composition for a golf ball where 5–42 parts by mass of trans-polybutadiene, 1–15 parts by mass of fine fiber having an amide group, and 5–30 parts by mass of natural rubber are blended in 100 parts by mass of a cover material containing trans-polyisoprene as a main component, as a cover material that realizes the best possible performance, that is, a flight distance of a multi-dimple type golf ball.
Japanese Patent Laying-Open No. 1-223980 proposes a thread-wound golf ball using a cover material where inorganic single crystal fibers are blended in a trans-1,4-polyisoprene-based cover material to improve a flight distance of the ball.
U.S. Pat. No. 5,716,293 discloses the use of a solid rubber center containing an oily material and a soft cover material to improve a shot feel and to increase a spin rate with a short iron as well. Since this technique uses an oil proof rubber or an ionomer resin having a high hardness for the outer side of the solid rubber center, the rebound performance and the shot feel is yet to be improved.
U.S. Pat. No. 5,935,021 proposes a technique to improve rebound property, durability and cut resistance by blending fibrous aluminum borate whiskers in a main material of a thermoplastic resin or a thermoplastic elastomer. This technique, however, reduces the rebound performance of the cover material by blending the aforementioned whiskers.
Japanese Patent Laying-Open No. 10-179802 proposes a golf ball where a base resin of a cover is formed of a two-component heated mixture, as a main component, of an ionomer resin and a styrene-butadiene-styrene block copolymer having a polybutadiene block containing an epoxy group or a styrene-isoprene-styrene block copolymer having polyisoprene block containing an epoxy group, characterized in that the composition forming the cover has a flexural modulus of 50–300 MPa and a Shore D hardness of 40–60. This technique is intended to improve the shot feel, the spin performance and the flight performance, but the cut resistance is still to be improved.
Japanese Patent Laying-Open No. 10-225532 proposes a cover composition for a golf ball formed of a resin composition in which aluminum borate whiskers are blended in a resin material made of an ionomer resin as a base material to improve durability.
Japanese Patent Laying-Open No. 6-319832 proposes a composition as a cover material formed by blending a core shell polymer made of (a) a core of a rubber-like polymer having an epoxy group or a carboxyl group or an acid anhydride group on the surface and (b) a shell of a glassy polymer, into an ethylene-unsaturated carboxylic acid copolymer, to improve a soft feeling and durability as well as rebound performance.
The Journal of the Society of Rheology, Japan, Vol. 25 (1997), pp. 275–282 has reported a development of a micro disperse system super-fine fiber reinforced composite prepared from a rubber-polyolefin-nylon ternary graft copolymer as an application of the plastic field.
These conventional techniques cannot improve the rebound performance, the spin performance, the cut resistance, durability, and feeling on the whole.